It is known to apply diffraction gratings and similar optically detectable microstructures to security documents or similar articles, such as identity cards, passports, credit cards, bank notes, cheques and the like. Such microstructures have the advantages of being difficult to falsify or modify, and being easily destroyed or damaged by any attempts made to tamper with the document. Accordingly, such optically detectable structures may be used to provide an effective security feature.
One common method of applying diffraction gratings and similar structures to security documents involves the use of multilayer thin films. The thin film devices are typically supported on carrier structures during production, and then transferred from the carrier substrate to the security document or other article, typically by using a hot stamping process.
Thin film diffractive structures may be employed usefully as security features on documents or other articles. However, they have a number of associated disadvantages, including the complex and highly specialised production process required, the requirement for an additional process step to transfer the thin film structure to the security document, and a relatively high susceptibility to physical wear and tear and/or chemical attack.
In order to mitigate these problems, one known alternative method of producing optically detectable structured devices involves the exposure of a substrate to laser radiation. According to this method, the substrate is transparent to visible light, but absorbs light at the wavelength of the laser, such that the exposure of the substrate results in ablation of the surface to form a three dimensional optically diffractive structure thereon.
Following ablation, the surface may be coated with a reflective film, to produce a diffractive optical structure that is visible in reflection through the transparent substrate. Alternatively, the surface may be left uncoated, or be coated with a transparent coating having a different refractive index to that of the substrate. According to this method, a diffractive optical element can be formed that is visible in transmission through the document, when illuminated using a point light source, such as a visible laser, projected onto a suitable viewing surface.
While this latter method of producing a diffractive optical element in a security document mitigates many of the problems inherent in the use of multilayer thin films, it is not applicable where it is desired to coat or laminate the transparent substrate with a protective layer of transparent material having a similar refractive index to that of the substrate. Lamination of the security document by protective layers is generally required for products such as passports and identity cards, while for non laminated products, such as bank notes, it is desirable to apply a protective varnish to the optically detectable microstructure. However, since the diffraction effect relies on a change in refractive index across the interface of the microstructure, the application of a protective layer of similar refractive index, and which generally fills the structure, is destructive of the diffractive effect.
Accordingly, it is desirable to provide a method of producing a diffractive optical microstructure in a security document or similar article that retains the diffractive effect even when covered and filled by a protective laminate or coating of similar refractive index.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art on or before the priority date of the claims appended hereto.